SE has been employed as additives for various products including foods, cosmetics, drugs, dish detergents, feeds and resins, because of its excellent surface-activating performance, biodegradability and stability. In the field of the chemical industry, these compounds are highly useful as reaction aids in, for example, polymerization and oxidization.
Examples of a known method for producing an SE include one described in JP-B-35-13102 (the term "JP-B" as used herein means an "examined Japanese patent publication"), wherein sucrose undergoes an ester exchange reaction with a fatty acid ester such as a fatty acid methyl ester in a reaction solvent such as N,N-dimethylformamide or dimethyl sulfoxide (hereinafter referred to simply as DMSO) in the presence of an alkali catalyst.
The reaction mixture obtained by the above-mentioned method contains the reaction solvent, the unreacted sucrose, the alkali catalyst, etc. in addition to the SE product. There have been proposed several methods for separating the SE from this mixture (as described in JP-B-48-21927, JP-B-48-35049, JP-A-50-29417, JP-A-50-130712, etc.; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Usually, the mixture is subjected to liquid-liquid extraction with the use of an organic solvent (for example, hexane, butyl alcohol, methyl ethyl ketone, ethyl isobutyl ketone) and water. Thus the SE migrates mainly into the organic solvent phase, while the unreacted sucrose and the reaction solvent migrate into the aqueous phase. The organic solvent phase is then separated from the aqueous phase and the organic solvent is removed from the organic solvent phase containing the SE by, for example, distillation. Thus the SE can be recovered.
However the above-mentioned method can hardly be put into industrial use, because it suffers from a number of problems including the occurrence of hydrolysis of the SE in the liquid-liquid extraction, the migration of the SE into the aqueous phase, and the insufficient formation of the interface in the extraction. Also, it is difficult to obtain a highly pure SE by this method.